A simplified purification procedure of mung bean nuclease has resulted in a stable, homogeneous enzyme. The enzyme is a glycoprotein of polypeptide molecular weight 39,000. About 70 percent of the molecules contain a peptide bond cleavage in a single region of the molecule resulting in two polypeptide chains, 25,000 and 15,000 daltons, held together by a disulfide bond(s). No activity (DNase, RNase, 3'-AMPase) was detected in the isolated, reoxidized fragments obtained after reduction of the cleaved form of the enzyme. The single-strand specific nuclease from mung beans possesses an intrinsic end preferring activity towards native DNA resulting in the formation of mono- to trinucleotides at all stages of the reaction. This activity is a manifestation of the single-strand specific endonuclease activity at the loose or partially denatured ends of native DNA. Mung bean nuclease cleaves native, viral DNA at a limited number of sites resulting in high molecular weight duplex fragments. Further characterization of these fragments as well as the study of their rates of formation is in progress. BIBLIOGRAPHIC REFERENCES: Kowalski, D. and Laskowski, M., Sr. Functional Characterization of Nucleophosphodiesterases, Handbook of Biochemistry and Molecular Biology, 3rd Edition, Fasman, G.D., edt., in press.